This is my lisp code. I’m trying to solve this problem:
Define function f3 that takes a simple list of integers as an argument and returns the number (count) of integers in the list which are between -3 and +15 (-3 and +15 included). For example:
LISP> (f3 '(1 -2 17 -4)) 2
This is my take on the problem and I’m stuck. I don't know what I’m doing wrong.
(defun f3 (x)
(if (null x)
0
(if (>(car x) - 3)
(if (<(car x) 15)
(+ 1(f3 (cdr x)))
(f3 (cdr x))
(f3 (cdr x))))))
You don't have enough parentheses after the second (f3 (cdr xx)). As you can see when the code is properly indented, that puts the last 2 recursive calls inside the same if; the last one was supposed to be in the previous if.
But there's no need for separate if expressions for -3 and 15. You can combine the tests with and, or take advantage of the fact that it's the same comparison for both endpoints and use (<= -3 (car x) 15). Note also that the question says that the endpoints are included, so you should use <= rather than <.
Finally, there shouldn't be a space between - and 3. I suspect that was just a copying error, since you would have gotten a different error in that case.
(defun f3 (x)
(if (null x)
0
(if (<= -3 (car x) 15)
(+ 1 (f3 (cdr x)))
(f3 (cdr x)))))